Is it possible to tap into Entity Framework and do this conversion behind the scenes?

Also, this feature is to be added to an already existing code base so I don't really want to have to add a couple of new properties in a partial class because it would require having to update every usage of the already existing properties. And even if I did that then I'd still have the problem of stopping users from using the wrong versions.

Also if I could push the transformation further down into EF then I wouldn't have to worry about the perfomance hit of encrypting/decrypting every time a user users one of the properties.

Answers

You can use Partial Classes to accomplish that. There are several valuable resources on how to use use partial classes to extend / customize your objects. For example :